diff --git a/dvc/config.py b/dvc/config.py
index b0372fd578..e8fd3e956c 100644
--- a/dvc/config.py
+++ b/dvc/config.py
@@ -246,7 +246,7 @@ class Config(object):  # pylint: disable=too-many-instance-attributes
 
     def __init__(self, dvc_dir=None, validate=True):
         self.dvc_dir = dvc_dir
-        self.validate = validate
+        self.should_validate = validate
 
         if not dvc_dir:
             try:
@@ -376,14 +376,10 @@ def load(self):
             c = self._lower(c)
             self.config.merge(c)
 
-        if not self.validate:
+        if not self.should_validate:
             return
 
-        d = self.config.dict()
-        try:
-            d = self.COMPILED_SCHEMA(d)
-        except Invalid as exc:
-            raise ConfigError(str(exc)) from exc
+        d = self.validate(self.config)
         self.config = configobj.ConfigObj(d, write_empty_values=True)
 
     def save(self, config=None):
@@ -421,6 +417,12 @@ def _save(config):
                 raise
         config.write()
 
+    def validate(self, config):
+        try:
+            return self.COMPILED_SCHEMA(config.dict())
+        except Invalid as exc:
+            raise ConfigError(str(exc)) from exc
+
     def unset(self, section, opt=None, level=None, force=False):
         """Unsets specified option and/or section in the config.
 
@@ -485,6 +487,11 @@ def set(self, section, opt, value, level=None, force=True):
             )
 
         config[section][opt] = value
+
+        result = copy.deepcopy(self.config)
+        result.merge(config)
+        self.validate(result)
+
         self.save(config)
 
     def get(self, section, opt=None, level=None):
diff --git a/tests/func/test_config.py b/tests/func/test_config.py
index f6cae78d3c..7fa4a45718 100644
--- a/tests/func/test_config.py
+++ b/tests/func/test_config.py
@@ -1,6 +1,8 @@
+import pytest
 import configobj
 
 from dvc.main import main
+from dvc.config import Config, ConfigError
 from tests.basic_env import TestDvc
 
 
@@ -33,11 +35,11 @@ def test_root(self):
         self.assertEqual(ret, 0)
 
     def _do_test(self, local=False):
-        section = "setsection"
-        field = "setfield"
+        section = "core"
+        field = "analytics"
         section_field = "{}.{}".format(section, field)
-        value = "setvalue"
-        newvalue = "setnewvalue"
+        value = "True"
+        newvalue = "False"
 
         base = ["config"]
         if local:
@@ -83,3 +85,13 @@ def test_non_existing(self):
 
         ret = main(["config", "core.non_existing_field", "-u"])
         self.assertEqual(ret, 251)
+
+
+def test_set_invalid_key(dvc):
+    with pytest.raises(ConfigError, match=r"extra keys not allowed"):
+        dvc.config.set("core", "invalid.key", "value")
+
+
+def test_merging_two_levels(dvc):
+    dvc.config.set('remote "test"', "url", "https://example.com")
+    dvc.config.set('remote "test"', "password", "1", level=Config.LEVEL_LOCAL)
diff --git a/tests/func/test_remote.py b/tests/func/test_remote.py
index 97a471a51a..6ebfc19edd 100644
--- a/tests/func/test_remote.py
+++ b/tests/func/test_remote.py
@@ -32,7 +32,7 @@ def test(self):
         self.assertEqual(main(["remote", "list"]), 0)
 
         self.assertEqual(
-            main(["remote", "modify", remotes[0], "option", "value"]), 0
+            main(["remote", "modify", remotes[0], "checksum_jobs", "1"]), 0
         )
         self.assertEqual(main(["remote", "remove", remotes[0]]), 0)
 
@@ -113,7 +113,7 @@ def test(self):
 
 class TestRemoteRemove(TestDvc):
     def test(self):
-        ret = main(["config", "core.jobs", "1"])
+        ret = main(["config", "core.checksum_jobs", "1"])
         self.assertEqual(ret, 0)
 
         remote = "mys3"